(1) Field of the Invention
The present invention generally relates to semiconductor micromachined devices. More particularly, this invention relates to a process of making an all-silicon capacitive microphone, in which a membrane used to sense sound vibration is formed of substantially stress-free single-crystal silicon bonded to a support substrate.
(2) Description of the Related Art
There is a continuing desire for audio devices that are smaller in size, lower in cost, and can be manufactured using high-volume manufacturing practices, yet are characterized by high reliability and sensitivity. An example is acoustic transducers such as microphones that make use of a silicon sensing membrane, examples of which are disclosed in U.S. Pat. No. 5,146,435 to Bernstein, and U.S. Patent Application Publication No. 2001/0015106 to Aigner et al. In both Bernstein and Aigner et al., the silicon sensing membrane is movable and capacitively coupled to a stationary silicon membrane, such that sound waves impinging on the silicon sensing membrane are sensed by changes in the capacitive output of the device.
Processes for fabricating all-silicon microphones of the type disclosed by Bernstein and Aigner et al. are typically long, cumbersome, expensive, and not compatible with high-volume processes. In addition, the silicon sensing membranes can be prone to process-induced deformation and package-induced stresses that can prevent or interfere with proper operation of the device. For example, the silicon membrane disclosed in Aigner et al. is formed of a deposited silicon film and capacitively coupled to a stationary membrane formed of epitaxially-grown silicon. As well known in the art, stresses in deposited films such as the silicon sensing membrane of Aigner et al. are difficult to control, and high temperature steps required to form and process the stationary membrane of Aigner et al. can lead to plastic deformation of surrounding structures, including the silicon sensing membrane. A further disadvantage of capacitive audio devices such as those taught by Bernstein and Aigner et al. is the difficulty with which the distance between the capacitively coupled membranes can be precisely predetermined. For example, the capacitive gap of Bernstein's crystal s device is established by the shape of the stationary silicon membrane, while in the device of Aigner et al. the capacitive gap is established by a deposited spacer layer.
In view of the above, there is a continuing need for a process of making a relatively low-cost all-silicon sound transducer that is compatible with high-volume manufacturing practices, yet yields a device characterized by high reliability and performance characteristics.